Settings redesign: grouped rail + sub-tabbed dense pages

[adibhanna]

What & why

The settings modal had grown overwhelming: 9 flat nav items (with jargon like MCP/CLI/Keymap sitting at the same level as Appearance), each carrying a 2-line description, plus a few very dense pages (Vault ~25 controls, Editor ~24). This reorganizes the experience — without removing a single setting or changing how anything persists.

Changes

Grouped rail. The 9 categories are grouped into Look & feel · Editing · Vault · System, each item now an icon + short label. Descriptions move out of the rail and onto the page header. An empty query renders the grouped rail; typing switches to the flat search-results list.

Sub-tabbed dense pages. The two heaviest pages become focused in-page sub-tabs:

• Vault → Location · Notes · System
• Editor → Vim · Search · Writing · Quick capture

Existing <Section>s are spliced into sub-tabs — no control is moved or rewritten.

Search-jump into sub-tabs. Clicking a search result now opens the sub-tab that owns it, then scrolls to and highlights the setting.

Keymap left as-is — its sticky live-filter + grouped list is already the right pattern for 50+ shortcuts; sub-tabs would only fragment that search. The lighter pages (Appearance, Typography, Templates, MCP, CLI, About) were already clean.

Guardrails (verified)

• No setting removed — control / search-anchor / binding counts are identical to main: 70 settingId, 22 settingsSearchTargetProps, 20 ToggleRow, 9 SegmentedRow, 16 TextInputRow, 3 SelectRow, 26 <Section>, 59 onChange.
• config.toml intact — every onChange/persist binding is preserved untouched, so anything a user hand-wrote into ~/.config/zennotes/config.toml still loads and round-trips (PORTABLE_PREF_KEYS controls all unchanged).
• Typecheck (app-core) ✓ · app-core tests (580 passing) ✓ · driven the real build over CDP to confirm the rail, both sub-tabbed pages, and search-jump render and behave correctly.

Notes

• The diff keeps the file's existing hand-formatted style. The repo has no prettier config, and running prettier would reformat ~1k unrelated lines in this file alone — so I kept the diff focused on the actual structural changes.
• Scoped to SettingsModal.tsx only.

[adibhanna]

Folded into the v2.8.0 release PR #269 (release/v2.8.0 -> main), which carries this settings work plus the performance pass. Closing in favor of the single release PR.