fix(mobile): make every session tab reachable in the tab strip

[Ark0N]

Fixes#257.

The problem

With five tabs open on a phone, the tabs on the right could not be reached. Two separate things caused it, and fixing either alone is not enough:

1. Selection never moved the strip.selectSession() goes through _updateActiveTabImmediate(), which toggles the .active class and nothing else. Nothing ever touched scrollLeft.
2. Every full rebuild reset the scroll to 0._fullRenderSessionTabs() replaces the strip's innerHTML, which drops scrollLeft back to 0. That rebuild fires on ambient events (a task badge appearing or disappearing, a session created or deleted elsewhere, a tab-order broadcast), so a strip the user had just swiped rightward snapped back to the first tab a moment later.

There was also a mobile-only reordering that hoisted the active session to the front of the strip, dating from when only one tab fit on a phone. It ran on full renders only, so with five tabs the order flipped depending on which render path happened to fire, the tab you just tapped teleported to position 0, and the Alt+N badge numbers were reshuffled.

The fix

• computeTabScrollLeft() in constants.js is a pure policy: given the strip's measurements and a tab's box it returns the scrollLeft that puts that tab in view (leaving a 16px sliver of the neighbour showing, so the strip still reads as scrollable), clamped to the scrollable range, and returns the current position when the tab is already visible.
• _scrollActiveTabIntoView() feeds it measured rects and applies the result. It writes the strip's own scrollLeft rather than calling scrollIntoView(), which also scrolls ancestors: on a phone that means the document, under a fixed header and possibly an open keyboard. It honours prefers-reduced-motion.
• _updateActiveTabImmediate() calls it, so every selection path (tap, swipe, Alt+N, command palette, notification) reveals its tab.
• _fullRenderSessionTabs() saves and restores scrollLeft across the rebuild, and re-reveals the active tab only when it actually changed (_lastRenderedActiveTabId). Restoring unconditionally and revealing conditionally is what lets someone browse the far end of the strip while a background render fires, without a newly selected tab ever being stranded off-screen.
• Mobile now renders tabs in sessionOrder, like every other device.
• overscroll-behavior-x: contain on the strip, so a swipe running past the last tab stays in the strip instead of chaining into the browser's back gesture.

Desktop is untouched: it wraps to a second row, scrollWidth === clientWidth, and the policy no-ops.

Tests

• test/tab-overflow.test.ts (runs in CI): six cases over the scroll-target math, covering both edges, the already-visible no-op, clamping at both ends, a tab wider than the window, and missing measurements.
• test/mobile/tabs.test.ts: five browser regressions on an iPhone 14 Pro profile with five real tabs rendered through _fullRenderSessionTabs(): reveal-on-select rightward and leftward, scroll preserved across an ambient rebuild, sessionOrder rendering on phones, and a real CDP touch drag reaching the last tab.

npm run test:ci passes locally (233 files, 4756 tests).

Note: test/mobile/tabs.test.ts has one pre-existing failure on master ("switching tabs with the keyboard closed...": it expects preserveKeyboard: false, while handleSessionTabClick passes no such option). That suite is excluded from CI so it drifted; this PR leaves it alone.