feat: move split-view pane controls into title-bar tabs

[Mng-dev-ai]

Summary

• Move per-pane mosaic chrome (title, maximize, close) into a new SplitTabs component pinned in the title bar; the mosaic toolbar now renders empty.
• Lift maximize/focus state into uiStore as maximizedTile / focusedTile, with a single focusTile action that also coarsens to activeAgentTile (primary vs secondary chat).
• Derive both ephemeral pointers on read — a tile id that's no longer a leaf is treated as absent — and clear maximizedTile at the canonical removeTileFromMosaic event so a re-added tile id can't resurrect a stale maximize.
• Reset all three pane pointers together on new chat, new secondary, and split close so the tab highlight, command targeting, and maximize never aim at a stale pane.
• Show tabs on both chat and landing splits; SplitTabs is self-sourcing, so agentTitles/onCloseTile props are dropped from SplitViewContainer and MosaicSplitView.

Test plan

• Split a chat into two panes — confirm both tabs render with correct titles in the title bar.
• Click a tab and click into a pane — focus highlight follows the active pane in both cases.
• Maximize a pane from its tab, then restore — split reappears with all panes' state intact.
• Close a pane (secondary, primary-with-secondary swap, and a non-agent tile) — correct pane closes and routing updates.
• Open a new chat / open and close a split — tab highlight and maximize reset, never pointing at the old chat.
• Open a workspace file on the landing page to create an editor pane — tabs appear there too.
• Verify tabs are hidden on mobile and for single-pane views.