feat: migrate AskBar input to Lexical to fix WKWebView caret drift

[quiet-node]

Overview

Replaces the AskBar's transparent-textarea + highlight-mirror input with a single Lexical-backed contentEditable. The two-layer overlay drifted the caret off the rendered glyphs in WKWebView once content wrapped or scrolled, because the textarea's wrapping geometry and the mirror's never stayed in perfect lockstep. A single editable element puts the caret in the same element as the text, so it can never drift.

What changed

• view/askbar/LexicalAskBarInput.tsx — the editor. ValueSyncPlugin is a two-way bridge between the host's canonical query string and the editor; BehaviorPlugin wires popover key interception (Enter / Arrows / Tab / Escape) and image paste; the contentEditable is plain-text only.
• view/askbar/CommandNode.ts — a TextNode subclass that renders a recognized slash-command trigger (e.g. /search) in violet inline, modeled on the canonical @lexical/hashtag node.
• view/askbar/commandMatch.ts — pure, word-boundary-aware matcher that finds the earliest known command trigger in a string. Drives the inline highlight via registerLexicalTextEntity.
• view/AskBarView.tsx — swaps the textarea for LexicalAskBarInput; slash-popover filtering and selection state are unchanged.
• App.tsx / App.css — inputRef is now a div; the manual textarea height resets are gone (the contentEditable auto-sizes via CSS, capped at 144px); window-drag handling now treats any [contenteditable="true"] region as interactive so drag-select works inside the input.

How it works

The host owns the canonical query. After any editor-originated edit the host value already equals the editor text, so the down-sync is a no-op and the caret stays put; it only rewrites the editor for genuine external changes (command completion, clear-on-submit). Those external rewrites commit with discrete: true and are guarded by a flag so they never echo back as a spurious change, which keeps the editor in lockstep with the host during rapid clear→restore cycles (submit-then-cancel).

Slash-command triggers highlight inline as a Lexical text entity: matched text is swapped into a CommandNode as you type and back to plain text when it stops matching, so the violet token tracks the native caret with no overlay layer.

Testing

• bun run test:coverage — 1549 tests pass, 100% line/branch/function/statement coverage, including dedicated suites for LexicalAskBarInput, CommandNode, and commandMatch.
• bun run typecheck, bun run lint:frontend, bun run format:check — clean.
• bun run build:frontend — succeeds.

Note: the input is plain-text only with no innerHTML path, so the migration adds no new markup-injection surface; paste remains capped by image size and count.