fix(mobile): pin CJK fonts so iOS WKWebView doesn't render ? boxes

[attson]

Repro

On iOS 26 Capacitor build (verified iPhone 17 Pro / iOS 26.3 simulator), all Chinese-locale i18n strings render as missing-glyph `[?]` boxes — `mobile.relayDisconnected` = "Relay 已断开" shows as "Relay [?] [?] [?]", same for `mobile.setupSubtitle`, all the insecure-HTTP warning text, etc. English text renders fine.

Root cause

iOS 26 WebKit regression. `-apple-system` (and `system-ui`) declares full Unicode coverage but doesn't actually render CJK glyphs, AND WebKit refuses to fall through to later families in the font-family list. Stacks that start with `-apple-system, ..., "PingFang SC", ...` fail every CJK glyph even with PingFang SC explicitly in the list.

Verified via a font-probe page hitting the live WKWebView:

stack	CJK render
`sans-serif` alone	✓
`-apple-system` alone	✗
`system-ui` alone	✗
`"PingFang SC"` direct	✓
`"Heiti SC"` direct	✓
`-apple-system, ..., "PingFang SC", ...` (the obvious "fix")	✗
`"PingFang SC", ..., Helvetica, sans-serif` (this PR)	✓

`document.fonts.check` reports all CJK families as present — so the fonts are installed, WebKit just won't reach them when `-apple-system` sits in front.

Fix

• New CSS vars `--font-sans` / `--font-mono` at `:root` in `src/style.css` with PingFang SC first, then Hiragino Sans GB / Heiti SC / Microsoft YaHei / Noto Sans CJK SC, then Helvetica Neue / Helvetica / Arial / sans-serif. `-apple-system` and `system-ui` are intentionally dropped — too risky given the iOS 26 bug, and Helvetica Neue is iOS's native Latin fallback already.
• Inline `<style>` in `index.capacitor.html` with the same stack so WKWebView's parse-time font-selection is correct before the Vue bundle's style.css loads.
• `` (was `"en"`) — i18n already syncs this on locale change but the parse-time value matters for WKWebView.
• All mobile component scoped styles (`MobileSetup`, `MobileSessionList`, `MobileTerminal`, `PairingConsume`) now reference `var(--font-sans|mono)` instead of repeating their own font stacks — future components inherit the fix automatically.

Test plan

• `npm test` — 644 tests green
• `npx vue-tsc --noEmit` — clean
• `npm run build:capacitor` — clean
• Font probe: row H ("PingFang SC first" stack) renders Chinese + English correctly on iOS 26.3 simulator
• manual: `cd mobile && npm run ios:sync && cap open ios`, Clean Build Folder, Run — confirm MobileSetup / MobileSessionList / MobileTerminal Chinese text renders.

Out of scope

• iOS < 26 / macOS Wails are not affected by the `-apple-system` regression; this fix only makes the desktop stacks slightly less Apple-native (PingFang SC's Latin glyphs are used instead of SF). Acceptable since the same bundle ships everywhere.
• Bundling a webfont (e.g. Noto Sans SC subset) was avoided — adds ~300KB-2MB and isn't needed once the system fonts are reachable.