refactor: compact session list row layout

[jongio]

Summary

Tightens the session list row layout by merging the check and cursor columns into a single 2-char selector, reducing tree-mode indent from 4 to 2 spaces, and making the host icon a fixed-width column. Saves 4 chars of prefix space, giving more room for summary text.

Changes

• Merge checkMark (2 chars) + indicator (2 chars) into single selector (2 chars)
• Reduce tree-mode indent from 4 to 2 spaces
• Make host icon a fixed 2-char column (was variable 0 or 2), fixing text misalignment between rows
• Extract applyRowStyle helper to eliminate duplicated style-application blocks
• Add tests for narrow-width rendering, tree-mode indent, and selector state transitions
• Update indirect dependencies (charmbracelet/ultraviolet, charmbracelet/x/exp/slice)

Type of Change

• Refactoring (no functional changes)

Testing

• Unit tests added/updated
• 13/13 packages passing, 0 failures

Quality Gates

Gate	Status	Notes
mq	PASS	Full 4-wave pipeline, 0 CRITICAL/HIGH
cr	PASS	Opus 4.6, 3 LOW (all fixed)
pf	PASS	vet + build + tests clean
secops	PASS	STRIDE clean, no attack surface
th	PASS	92.2% package coverage