Add a Gemini-style AI side panel and unify the dashboard's chat and buttons

[encoreshao]

Summary

A Gmail/Gemini-style assistant panel, available from every dashboard page, plus a consistency pass on the Dashboard chat and buttons.

AI side panel

• Topbar AI button (first in the right-hand group) opens a rounded panel to the right of the main card; the main card shifts over to make room.
• Resizable: drag the panel's left edge (320–720px), or use ←/→ (Shift for bigger steps) and Home/End; double-click resets to 400px. Below 720px it overlays instead and isn't resizable.
• Persistent: open state, width and the panel's chat session survive page navigation and auto-refresh reloads (restored before first paint, so no flicker).
• Page-aware prompts: each page offers its own 2–4 suggested prompts (e.g. Memory → "What has the loop learned so far?"). Read-only prompts send immediately; prompts that would start a run only pre-fill the composer.
• Page context: POST /activity/chat accepts an optional page, so the assistant knows what the user is looking at. Only known nav keys are used; any other value is ignored.
• Chat history inside the panel lists every session, whether started in the panel on any page or on the Dashboard, and reopens one in place.
• Reuses the existing chat backend (/activity/chat + /activity/chat-stream SSE). No new state-changing routes.

Chat experience (panel and Dashboard)

• Shared "Thinking…" indicator: an AI sparkle in a spinning gradient ring, a shimmering label and placeholder lines. Like .md-spinner, it's not gated behind prefers-reduced-motion, since it's the only sign a reply is coming.
• Paced streaming reveal: the CLI often emits a short reply's deltas within ~1s after a long think, which read as "no streaming". Both chats now reveal received text progressively, catching up a burst in about 2 seconds, and swap in the saved markdown reply only after that. Hidden tabs and reduced-motion users get the text immediately.
• Reply bubbles show the Loop X icon (and user bubbles "You") while streaming, not only after the reload.

Visual consistency

• Dashboard History / New chat buttons, suggestion chips and hero links now use the global .btn .btn-neutral style.
• Links carrying an icon align the glyph with their text (zero-specificity :where() rule, so component styles still win). The Dashboard announcement link uses an arrow icon instead of a text →.
• Topbar right-group controls share a 40px height.
• Fix: the empty Dashboard composer drifted off-center when the panel was open (a leftover right offset under position: relative).

i18n

All new strings are translated into Japanese, Simplified Chinese and French.

Test plan

• python3 -m pytest tests/ -q: 1861 passed
• New tests: panel markup and trigger, per-page prompts (coverage of every nav page, translations, icon subset, run prompts only pre-fill), page context in build_chat_prompt and the route, chats from any page landing in history, shared thinking indicator, reveal wiring, Dashboard .btn classes, composer centering, topbar control heights
• Sandboxed dev server (LOOP_ENGINEERING_HOME scratch dir) driven with headless Chrome: open/close, drag and keyboard resize, persistence across navigation, history reopen, Escape handling, Japanese and narrow-viewport layouts, thinking → streaming → saved-reply transition. Chat requests were stubbed at the network layer, so no real messages were saved.