docs: fix misc remaining docs (architecture, dev quickref, sub-dir READMEs)

PLUGIN_ARCHITECTURE_SPEC.md
- Added a banner at the top noting this is a historical design doc
  written before the plugin system shipped. The doc is ~1900 lines
  with 13 stale /api/plugins/* paths (real is /api/v3/plugins/*),
  references to web_interface_v2.py (current is app.py), and a
  Migration Strategy / Implementation Roadmap that's now history.
  Banner points readers at the current docs
  (PLUGIN_DEVELOPMENT_GUIDE, PLUGIN_API_REFERENCE,
  REST_API_REFERENCE) without needing to retrofit every section.

PLUGIN_CONFIG_ARCHITECTURE.md
- 10 occurrences of /api/plugins/* missing /v3 prefix. Bulk fixed.

DEVELOPER_QUICK_REFERENCE.md
- cache_manager.delete("key") -> cache_manager.clear_cache("key")
  with comment noting delete() doesn't exist. Same bug already
  documented in PLUGIN_API_REFERENCE.md.

SSH_UNAVAILABLE_AFTER_INSTALL.md
- 4 occurrences of port 5001 -> 5000 in AP-mode and Ethernet/WiFi
  recovery instructions.

PLUGIN_CUSTOM_ICONS_FEATURE.md
- Port 5001 -> 5000.

CONFIG_DEBUGGING.md
- Documented /api/v3/config/plugin/<id> and /api/v3/config/validate
  endpoints don't exist. Replaced with the real endpoints:
  /api/v3/config/main, /api/v3/plugins/schema?plugin_id=,
  /api/v3/plugins/config?plugin_id=. Added a note that validation
  runs server-side automatically on POST.

STARLARK_APPS_GUIDE.md
- "Plugins -> Starlark Apps" UI navigation path doesn't exist (5
  occurrences). Replaced with the real path: Plugin Manager tab,
  then the per-plugin Starlark Apps tab in the second nav row.
- "Navigate to Plugins" install step -> Plugin Manager tab.

web_interface/README.md
- Documented several endpoints that don't exist in the api_v3
  blueprint:
  - GET /api/v3/plugins (list) -> /api/v3/plugins/installed
  - GET /api/v3/plugins/<id> -> doesn't exist
  - POST /api/v3/plugins/<id>/config -> POST /api/v3/plugins/config
  - GET /api/v3/plugins/<id>/enable + /disable -> POST /api/v3/plugins/toggle
  - GET /api/v3/store/plugins -> /api/v3/plugins/store/list
  - POST /api/v3/store/install/<id> -> POST /api/v3/plugins/install
  - POST /api/v3/store/uninstall/<id> -> POST /api/v3/plugins/uninstall
  - POST /api/v3/store/update/<id> -> POST /api/v3/plugins/update
  - POST /api/v3/display/start/stop/restart -> POST /api/v3/system/action
  - GET /api/v3/display/status -> GET /api/v3/system/status
- Also fixed config/secrets.json -> config/config_secrets.json
- Replaced the per-section endpoint duplication with a current real
  endpoint list and a pointer to docs/REST_API_REFERENCE.md.
- Documented that SSE stream endpoints are defined directly on the
  Flask app at app.py:607-615, not in the api_v3 blueprint.

scripts/install/README.md
- Was missing 3 of the 9 install scripts in the directory:
  one-shot-install.sh, configure_wifi_permissions.sh, and
  debug_install.sh. Added them with brief descriptions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/STARLARK_APPS_GUIDE.md

@@ -91,7 +91,7 @@ Pixlet is the rendering engine that executes Starlark apps. The plugin will atte
 
 #### Auto-Install via Web UI
 
-Navigate to: **Plugins → Starlark Apps → Status → Install Pixlet**
+Navigate to: **Plugin Manager → Starlark Apps tab (in the second nav row) → Status → Install Pixlet**
 
 This runs the bundled installation script which downloads the appropriate binary for your platform.
 
@@ -110,10 +110,10 @@ Verify installation:
 
 ### 2. Enable the Starlark Apps Plugin
 
-1. Open the web UI
-2. Navigate to **Plugins**
-3. Find **Starlark Apps** in the installed plugins list
-4. Enable the plugin
+1. Open the web UI (`http://your-pi-ip:5000`)
+2. Open the **Plugin Manager** tab
+3. Find **Starlark Apps** in the **Installed Plugins** list
+4. Enable the plugin (it then gets its own tab in the second nav row)
 5. Configure settings:
    - **Magnify**: Auto-calculated based on your display size (or set manually)
    - **Render Interval**: How often apps re-render (default: 300s)
@@ -122,7 +122,7 @@ Verify installation:
 
 ### 3. Browse and Install Apps
 
-1. Navigate to **Plugins → Starlark Apps → App Store**
+1. Navigate to **Plugin Manager → Starlark Apps tab (in the second nav row) → App Store**
 2. Browse available apps (974+ options)
 3. Filter by category: Weather, Sports, Finance, Games, Clocks, etc.
 4. Click **Install** on desired apps
@@ -307,7 +307,7 @@ Many apps require API keys for external services:
 **Symptom**: "Pixlet binary not found" error
 
 **Solutions**:
-1. Run auto-installer: **Plugins → Starlark Apps → Install Pixlet**
+1. Run auto-installer: **Plugin Manager → Starlark Apps tab (in the second nav row) → Install Pixlet**
 2. Manual install: `bash scripts/download_pixlet.sh`
 3. Check permissions: `chmod +x bin/pixlet/pixlet-*`
 4. Verify architecture: `uname -m` matches binary name
@@ -338,7 +338,7 @@ Many apps require API keys for external services:
 **Symptom**: Content appears stretched, squished, or cropped
 
 **Solutions**:
-1. Check magnify setting: **Plugins → Starlark Apps → Config**
+1. Check magnify setting: **Plugin Manager → Starlark Apps tab (in the second nav row) → Config**
 2. Try `center_small_output=true` to preserve aspect ratio
 3. Adjust `magnify` manually (1-8) for your display size
 4. Some apps assume 64×32 - may not scale perfectly to all sizes
@@ -349,7 +349,7 @@ Many apps require API keys for external services:
 
 **Solutions**:
 1. Check render interval: **App Config → Render Interval** (300s default)
-2. Force re-render: **Plugins → Starlark Apps → {App} → Render Now**
+2. Force re-render: **Plugin Manager → Starlark Apps tab (in the second nav row) → {App} → Render Now**
 3. Clear cache: Restart LEDMatrix service
 4. API rate limits: Some services throttle requests
 5. Check app logs for API errors