Fixes #214

## Summary

The chat rail is the right-hand column and its resize handle sits on its left edge (`side="left"`). `resizeDelta("left", d)` returned `+d`, so moving the edge right **widened** the chat rail, and `ArrowRight` did the same. The handle moved opposite to the pointer.

`side` names the edge the handle sits on, so the mapping now follows the edge's geometry:

- left edge: moving right narrows the pane (`-delta`) - right edge: moving right widens the pane (`+delta`)

`keyboardPaneDelta` goes through `resizeDelta`, so the arrow keys now move the separator in the arrow's direction too. The Projects sidebar's `SidebarResizeHandle` uses its own raw `clientX` delta and was already correct, so it is unchanged.

## Tests

- `apps/web/src/resizable-pane.test.ts`: replaces the assertions that pinned the inverted mapping with ones for both edges, for pointer and keyboard input. - `e2e/shell.e2e.ts`: "the chat rail has its own resize control" now widens with `ArrowLeft` and narrows with `ArrowRight`. The new test "the chat rail edge follows the pointer" drags the edge left (rail widens), then right (rail narrows).

Both e2e tests fail against the old `resizeDelta` and pass with the fix.

Assistant-model: Claude Opus 5.5 Assistant-workflow: inline Assistant-duration: 4m converged, estimated 15m Assistant-verification: bun test unit passed: apps/web/src/resizable-pane.test.ts (5 pass) Assistant-verification: Playwright Chromium E2E passed: bun run e2e e2e/shell.e2e.ts (11 passed) Assistant-verification: Playwright Chromium E2E regression check passed: both chat-rail resize tests fail against the old resizeDelta Assistant-verification: typecheck and lint passed: bun run types, bun run ci Co-authored-by: Alex Lavaee <lavaman131@github.com>

Reviewed the resize geometry and keyboard mapping for both pane edges. Dragging the chat rail’s left edge left now widens it, and the arrow keys follow the same screen direction. No blocking issues found.

Reran the focused resize tests: 5 passed, 0 failed. The PR includes pointer and keyboard browser regression coverage; validation, browser integration, and container CI all pass.