feat(tui): two-pane focus navigation and mouse on select screen

[fullstackjam]

What does this PR do?

Reworks the install wizard's SELECT screen from "← → cycles categories, ↑ ↓ always drives the list" into an explicit two-pane focus model, and adds mouse support.

Why?

The screen paired a category sidebar with a package list but had one confusing input model — two columns, yet ← → cycled categories while ↑ ↓ only ever moved the list. Now:

• ← → (and tab) move focus between the sidebar and the list; the active pane's cursor is bright, the other dimmed.
• ↑ ↓ act on the focused pane — change category (list live-previews) under sidebar focus, move the package cursor under list focus.
• Mouse: left-click a category to switch to it, left-click a package to toggle it, wheel to scroll the list (via WithMouseCellMotion).

Visual style is unchanged — interaction only.

Testing

• go vet ./... passes
• Relevant tests added/updated — unit tests for the focus model, the pure selectHitTest click→row mapping, mouse click/wheel handling, and a focus-render assertion (TestSelectFocusIsVisuallyIndicated, which forces a colour profile since lipgloss strips colour without a TTY)
• New L3 pty test TestE2E_InstallWizard_MouseTogglesPackage drives a real terminal, sends an SGR mouse-click escape sequence, and asserts the toggle — ran green locally alongside the existing keyboard choreography
• Tested locally by driving the wizard on a tmux pty (isolated HOME, quit before confirm — nothing installed): ← → focus, ↑ ↓ per-pane, mouse category click, and select-all all verified

Cross-repo checklist

• Does this need a docs/content update in openboot.dev? — No
• Does this change the CLI ↔ server API contract? — No

Notes for reviewer

• selectHitTest (click coordinate → row) is a pure function whose geometry mirrors the renderer, so the mapping is unit-tested rather than eyeballed.
• Known follow-up (deliberately out of scope): the focus indicator is colour-only, so it's invisible without colour (piped / colourblind). A structural cue (dim/hidden cursor on the unfocused pane) would fix that — flagged, not done here.
• go.mod: termenv moves from indirect to a direct dep (used by the colour-profile test).

https://claude.ai/code/session_01HLoiSd2k9EJJ1HPjjDgUgo