Add customizable sidebar navigation

[brsbl]

Human comments

What was wrong

Built-in navigation and plugin pages were owned by separate sidebar regions. Users could not choose which destinations stayed visible or arrange them as one coherent navigation list, and compact layouts had no matching customization flow.

What changed

• Uses one persisted order for built-in navigation and plugin pages.
• Adds a quiet Customize entry point on the New thread row.
• Opens Customize sidebar as a desktop popover and an in-place compact mode with Back.
• Lists every destination with the shared multi-select checkbox and a drag grip.
• Applies visibility and order immediately. Hidden plugin pages remain launchable from Customize.
• Keeps every destination's existing behavior, including modifier-click opening in a split.
• Keeps all built-ins visible by default and shows the first three optional plugin pages for users without saved preferences.
• Migrates legacy visibility preferences without overriding explicit choices.
• Preserves compact thread-list state while Customize is open.
• Leaves plugin-row action-menu cleanup to the next stack layer.

Screenshots

The before revision is parent PR #2915 at exact head 150e60aef. The after revision is this PR at exact head 9f5e741aa. Each comparison uses the same dev fixture and rendered viewport.

Before — parent #2915	After — unified navigation at rest

Desktop Customize — exact PR head

Mobile before — parent #2915	Mobile after — in-place Customize mode

How you verified

• Drove exact head 9f5e741aa in Chrome for Testing 151.0.7922.71 against the branch dev app.
• Verified the complete desktop flow at 1440×1000 and the compact flow at 390×500; captured comparable evidence at 340×900 and 390×844.
• Confirmed all built-ins remain visible when the saved order interleaves built-ins and plugins, while exactly three optional plugins are shown by default.
• Confirmed first-use reorder preserves checked membership and that visibility and order persist across reloads.
• Confirmed New thread, plugin pages, and Automations preserve modifier-click split behavior from Customize.
• Confirmed Automations retains its plugin-row options and accessory behavior.
• Confirmed compact Customize retains the same thread-list DOM node, scroll position, accessibility state, and focus when entering and returning with Back.
• Confirmed compact plugin launch navigates and closes the sidebar.
• Observed no browser exceptions, console errors, stale frames, or layout shifts in the changed flow.
• Remote CI passed on the exact head. Per repository policy, CI-equivalent checks were not run locally.

BB-Thread-ID: thr_ccffp4w2p2

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