feat: Add DankDash tab support for plugins

[jimallen]

Summary

This PR adds the ability for plugins to provide custom tabs in DankDash, enabling plugin authors to create rich, integrated experiences within the shell's dashboard.

Features

• New dashtab plugin type - Standalone tab plugins that only provide a DankDash tab
• tabComponent manifest field - Widget plugins can now also provide a DankDash tab alongside their DankBar widget
• Dynamic tab loading - Plugin tabs are automatically added/removed when plugins are enabled/disabled
• Tab positioning - Plugins can specify tabPosition: "start" or "end" to control placement
• Customizable appearance - tabName and tabIcon fields for tab bar display

Plugin Manifest Examples

Standalone tab plugin:

{
    "id": "myTab",
    "name": "My Tab",
    "type": "dashtab",
    "tabIcon": "dashboard",
    "component": "./MyTab.qml"
}

Widget plugin with optional tab:

{
    "id": "myWidget",
    "name": "My Widget", 
    "type": "widget",
    "component": "./MyWidget.qml",
    "tabComponent": "./MyTab.qml",
    "tabName": "My Tab",
    "tabIcon": "star"
}

Changes

QML (quickshell/):

• Services/PluginService.qml - Added pluginDashTabComponents, tab component loading/unloading, getDashTabPlugins() function
• Modules/DankDash/DankDashPopout.qml - Dynamic plugin tab rendering with proper index management

Go (core/):

• internal/plugins/manager.go - Extended pluginManifest struct with tab fields and helper methods
• internal/plugins/manager_test.go - Added comprehensive tests for tab manifest parsing

Documentation:

• docs/PLUGINS.md - New plugin system documentation including tab plugin examples

Test Plan

• make fmt - Code formatted
• make test - All tests pass (including new tab manifest tests)
• go mod tidy - Dependencies clean
• Manual testing with a plugin that provides a tab component

🤖 Generated with Claude Code

[bbedward]

This is flawed for a few reasons

1. No reason to change the plugin manager at all, it doesn't care about the contents besides for managing the plugins, the shell/qml side is the parser
2. Tab metadata like the name and icon don't belong in the manifest they belong in a Plugin component (like DesktopPluginComponent)
3. The dash itself is not customizable at all, that would probably be a pre requisite to plugins at all

I also wonder what kind of dash plugins are wanted at all, that wouldn't fit into the bar or control center. The dash is a special case where those 3 bar widgets have special behaviors when clicked unlike normal stuff - so there's a lot more to consider if there were plugins in general for it