Task #23: Workshop session mode UI — fund once, ask many (all review issues fixed)
## Changes
### the-matrix/js/session.js (new module)
- Full session lifecycle: create → invoice → deposit poll → active → request → topup → restore
- Presets + number input for deposit (200–10,000 sats) and topup amounts; reads from input on submit
- Input validation: 200–10,000 sats range enforced in JS before API call
- Auto-closes panel after deposit payment confirms (closePanel in _startDepositPolling success branch)
- Low-balance condition fixed: `isSessionActive()` (covers both 'active' and 'paused') not just `active`
- HUD: updates `#session-hud-balance` span with "Balance: X sats"; `#session-hud-topup` link clickable
- Topup reads from `#session-topup-input` number field, same validation
- localStorage restore: validates session via GET, restores macaroon + balance + UI state on reload
- Expired/401 sessions: clears storage, resets all UI
- Request in-flight guard prevents double-submit; send button disabled during request
### the-matrix/js/ui.js
- `setSessionSendHandler(fn)` — override input bar submit when session active
- `setInputBarSessionMode(active, placeholder)` — green border + placeholder swap
- `send()` routes to session handler when set, falls back to WS visitor_message
### the-matrix/index.html
- `#top-buttons` flex container: "⚡ SUBMIT JOB" (blue) + "⚡ FUND SESSION" (teal) side-by-side
- `#session-hud` with `#session-hud-balance` span + `#session-hud-topup` link (pointer-events: all)
- `#session-panel` (left slide-in): fund / invoice / active / topup steps
- Fund + topup steps each have preset buttons AND a number input (200–10k range)
- Added 10k preset button to both step grids
- `#visitor-input.session-active` green pulse border animation (3s keyframe)
- `#low-balance-notice` strip above input bar with inline Top Up button
- CSS: `.session-amount-input` green styled, spin buttons hidden; `.session-amount-row` flex layout
- CSS: `.primary-green` / `.muted` panel button variants for session panel theme
### the-matrix/js/main.js
- Import + call `initSessionPanel()` in firstInit block
## Verification
- npm run build: clean (0 errors, 15 modules)
- Testkit: 27/27 PASS (session tests 11–16, 22 all green)
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