[Feature]: allow right panel drag beyond 1200px to dynamic max

### Problem / opportunity

The right panel resizer is hard-capped at RIGHT_PANEL_CONFIG.MAX_WIDTH (1200px). On wide
windows (>=1604px container), dragging the right panel to widen it is clamped back to 1200px,
leaving the chat pane unable to use the available width. The user cannot widen the right panel
past one page even when there is room on screen.

### Proposed behavior

Allow the right panel to be dragged beyond 1200px up to a dynamic upper bound
(container - resizer - MIN_CENTER_WIDTH) while always reserving a 400px minimum chat column.
On wide windows the right panel can be widened past 1200px toward the available width; on
regular containers the dynamic upper bound still applies.

### Product surface

Web UI

### Details, examples, or constraints

- calculateValidRightWidth uses the pure dynamic max as the upper bound, dropping the
Math.min(RIGHT_PANEL_CONFIG.MAX_WIDTH, dynamicMax) hard cap.
- Remove the SessionScene aux-pane CSS max-width:1200px cap and de-cap updateRightPanelWidth
(clamp only to the compact minimum) so a wide manual drag is not pulled back to 1200px.
- Clamp visibilitychange and panel-expand restore paths to the dynamic max so a shrunken window
cannot transiently squash the chat pane below its one-page minimum.
- A 400px minimum chat column is preserved; default open/restore paths still cap at MAX_WIDTH.

Start by locating calculateValidRightWidth, RIGHT_PANEL_CONFIG, SessionScene, and updateRightPanelWidth. Trace the drag, visibilitychange, panel-expand, and default restore paths, then verify on wide and shrunken windows that manual resizing can exceed 1200px while the chat column stays at least 400px and default restores remain capped.

索引モデルが issue の本文から書いたものです。