fix: render path suggestions as floating overlay to prevent dialog jump

[oryaacov]

Summary

• New session dialog layout jump: When focusing/unfocusing the path field, the suggestions dropdown added ~3-7 lines inline, causing the entire dialog to resize and jump. This was disorienting when tabbing through fields.
• Fix: Render path suggestions as a floating overlay (z-index style) that paints on top of the dialog content instead of being part of the content flow. The dialog size stays constant regardless of whether suggestions are visible.

Changes

Commit 1: Overlay infrastructure (feat)

• overlayDropdown(base, overlay, row, col) — paints a multi-line string on top of a rendered TUI screen at a specific position, ANSI-aware
• truncateVisible / sliceVisibleFrom — ANSI-safe string slicing helpers for the overlay
• dropdownMenuBg() — returns a slightly elevated background color (dark: #292e42, light: #dcdde2) to distinguish the menu from the dialog surface
• renderSuggestionsDropdown() — renders path suggestions as a bordered floating menu with distinct background and keybinding footer
• 4 unit tests for the overlay helpers

Commit 2: Usage (fix)

• Remove inline suggestion rendering from both single-path and multi-repo code paths (eliminates duplicated ~55-line blocks)
• Track suggestionsLineOffset during content building to know where to position the overlay
• After lipgloss.Place centers the dialog, overlay the suggestions menu at the computed position

Visual behavior

Before	After
Dialog grows/shrinks when suggestions appear	Dialog stays fixed size
Suggestions are inline, push content down	Suggestions float over content below
No visual separation from dialog	Bordered menu with distinct background

Test plan

• All existing TestNewDialog_* tests pass (47 tests)
• New TestOverlayDropdown_* tests pass (4 tests)
• Full internal/ui test suite passes
• Manual: open new session dialog, tab to path field — suggestions appear without layout shift
• Manual: tab away from path field — suggestions disappear without layout shift
• Manual: verify dark and light themes both render the menu background correctly

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