feat: find any action by typing, with Ctrl+K

[JackCuthbert]

Closes #26.

Adds a command palette (Ctrl+K) that finds any action by typing, plus the nav and floating-button work that grew out of putting it on screen.

The palette

Navigation and app actions only — no per-todo actions, and no todo search, since the Search view already owns that. Results are grouped (Create / Go to view / Go to list / App) and each entry shows its chord where it has one. Ctrl+N / Ctrl+P move the selection as well as the arrows.

Commands are now a registry of their own (commands/lib/commands.ts) rather than an extension of the shortcut table. Not everything deserves a keybinding, and the previous shape forced one on anything that wanted to be listed. Shortcuts now reference a CommandId, so the two stay in sync without the registry dictating the keymap. Sign-out is deliberately excluded — it's in the footer, and a fuzzy match away from "Search" is not where you want to land.

Floating buttons

+ New todo and the palette square, reachable without opening the nav — on touch always, on desktop whenever the sidebar is collapsed. They sit inside mainScrollInner so they stay centred on the list while the sidebar animates, and sticky at the foot of the scroll port so a short list keeps them at the bottom of the screen rather than floating mid-page.

The palette button uses a terminal prompt rather than LuCommand: that glyph is ⌘, and this app binds Ctrl on every platform and refuses metaKey outright. An icon naming the one modifier the app never uses would be wrong on a Mac and meaningless everywhere else.

Sidebar rebalance

Adding a fourth button at the top made the nav read as three unrelated zones — chrome, then ghost-text views, then boxed-in lists. So the rows were brought onto one shape:

• List rows lose the segmented shell (border, shadow, inner divider) and take the same box as the views: one row shape, a soft wash for hover, and the same wash for selection.
• Group headings for Views and Lists, matching quick add's, which does the separating the dividers used to.
• New list moved to the foot of the lists it adds to, styled as a row in that group rather than a button parked underneath — italic, because it's an action about the group rather than a list in it.
• The filter moved into the Lists heading. A heading carries at most one button: two sat there briefly and read as a small toolbar that had drifted into a label. If a group ever needs more, that control becomes a menu instead of growing a neighbour.
• The kebab is now LuEllipsisVertical rather than the ⋮ character. As text it drew 7×16 of thin ink at weight 400 beside a 14px sparkle stroked at 2 — no size reconciles those, because one is type and the other a mark. Both are react-icons/lu now, per CLAUDE.md's one-icon-set rule.
• Long list names ellipsise. They never had: .link carried text-overflow: ellipsis but is a flex container, and that property only acts on a block's own inline content, so the declaration was inert and a 617px name ran on inside a 246px button.
• Icon buttons align on their glyph centres, not their box edges. The filter is 24px against the rows' 34px, so matching right edges left the glyphs 5px apart down the column.

Tooltips are new (ui/tooltip), on the filter and the palette button. title never appears for a keyboard user, can't be styled, and can't show a chord in the app's own type.

Notes for review

• Moving New list out of the heading changed its accessible name from + New list to New list, which broke 69 e2e tests — createList is used almost everywhere. The six call sites were updated rather than re-adding the label, since the accessible name should match the visible text.
• README screenshots regenerated, since the nav changed what they show.
• The list filter is deliberately not a command, so the filter button is the only route to it on touch. That's why both heading buttons stayed visible on mobile rather than being hidden as desktop-only. Worth deciding separately whether the filter should become a command.
• bun run test:e2e 78 passed, bun run test 751 passed, lint / typecheck / knip / fmt clean. Each commit typechecks and passes knip standalone.

Still open

The user guide (apps/docs/guide) has no command palette page yet. Happy to add it here or as a follow-up.