feat(desktop): add sidebar categories and manual channel ordering

[NYTEMODEONLY]

Summary

• promote existing custom channel sections into first-class sidebar categories
• add Manual alongside A-Z and Recent for exact channel ordering
• support pointer, touch, and keyboard movement within and across categories
• persist exact order in a separate encrypted channel-manual-order NIP-78 document
• preserve pending local category edits during startup/reconnect hydration
• keep draggable sidebar rows paintable in WKWebView by avoiding content-visibility: auto on transformed sortable rows
• hide channel drag handles at rest and reveal only the hovered, keyboard-focused, or actively dragged row, while keeping them visible on non-hover/touch devices

Why

Buzz already supported encrypted custom sections and section reordering, but category creation was buried in a channel context menu and exact channel order was not persisted. This extends the existing model rather than adding a competing category subsystem.

Behavior

• Create categories from the Channels actions menu.
• Rename, reorder, and safely delete categories.
• Deleting a category returns its channels to the uncategorized Channels group.
• Empty categories remain valid pointer and keyboard drop targets.
• Manual ordering supports same-category and cross-category insertion.
• Keyboard drag supports cancellation and live-region announcements.
• Drag handles stay mounted and accessible but are visually hidden at rest.
• Hovering anywhere on a draggable channel row reveals only that row's handle.
• Keyboard focus and active drag keep the handle visible.
• Non-hover/touch devices retain a persistent move affordance.
• Existing A-Z and Recent behavior remains unchanged.

Persistence and conflict handling

• Category membership remains in the existing encrypted channel-sections document.
• Exact channel order uses a separate encrypted channel-manual-order document so older clients cannot erase category membership or manual order.
• Stores are scoped by identity and normalized relay.
• Explicit pending local category edits stay authoritative while startup/reconnect hydration is in flight. Remote timestamps are still observed, then pending local state publishes strictly newer. Idle remote updates retain the existing timestamp/event-ID conflict rules.

Canary acceptance regressions fixed

Startup sync race

Creating a category before the initial NIP-78 fetch completed could show it briefly, then silently remove it. The older fetched snapshot overwrote local state and cancelled the pending debounced publish. The second commit adds the same pending-local guard used by manual-order sync and covers idle versus pending-local remote application.

Invisible rows during drag

In the installed macOS Canary, an ungrouped draggable row could remain in the DOM and accessibility tree but stop painting while dnd-kit transformed it. The affected row combined content-visibility: auto with sortable transforms; switching sort modes or cancelling the drag forced a repaint. The third commit removes that paint-skipping utility only from draggable channel rows, while retaining it for non-draggable rows. The browser regression pins contentVisibility === visible before and after a drop and checks that every channel remains visible.

Always-visible drag handles

The drag grip previously painted on every draggable channel row, adding persistent visual noise. The follow-up keeps the control mounted and tabbable, hides it with opacity at rest, and reveals it for row hover, focus-within, focus-visible, and active drag. A hover-capability media query leaves the control visible on non-hover/touch devices. The regression covers at-rest opacity, row-to-row hover transfer, keyboard focus and pickup, cancellation, pointer reorder, persistence, and row paint visibility.

Validation

Focused upstream branch at 2097ac40f88967d2bf3ed685f5d51a54ebd1561e:

• Desktop unit suite: 3,536 / 3,536 passed
• TypeScript/E2E build: passed
• category/manual-order browser flows: 5 / 5 passed after a fresh E2E build
• changed-file Biome, px-text, pubkey-truncation, and TypeScript checks passed
• repository-wide file-size check still reports the pre-existing desktop/src-tauri/src/managed_agents/runtime.rs limit (2,220 lines versus 2,216); this PR does not touch that file

Exact Canary integration at c4319dc4d06b6169a6f014c65f306a6c93ba2d9b:

• Desktop unit suite: 3,525 / 3,525 passed
• TypeScript/E2E build: passed
• category/manual-order browser flows: 5 / 5 passed in an isolated rerun
• changed-file Biome, px-text, pubkey-truncation, and TypeScript checks passed
• built and installed bundle ID: xyz.block.buzz.app.dev.canary
• built and installed executable SHA-256: 20ccbcb7836ff2eb4c81cd4b3d60cadc3e853c7b68c7cee858a0cfbf839cfcfb
• live installed WKWebView acceptance passed: handles were absent at rest; only the hovered row revealed its handle; hover transferred cleanly between rows; keyboard pickup kept the focused handle visible and announced its position; Escape restored state; Cmd-R and a full quit/relaunch preserved all eight visible and AX-reachable Channels rows in their original order
• no category, channel membership, or manual order was deleted, reset, or reassigned during acceptance