fix(sidebar): share compact rows and drag feedback

[JoeJoeflyn]

Description

Compact pinned sidebar rows by using the insertion gaps as the spacing instead of adding spacing around each gap. Show clear before/after reorder lines for both built-in places and pins, without letting the general file-drop outline override them. Folder-body transfers and spring navigation remain unchanged.

Visual evidence

screenrecording-2026-10-16_22-48-30.mp4

How to test

1. Pin several folders. Compare their spacing with Home/Trash, in both the full sidebar and icon rail.
2. Drag Home/Trash or a pinned row above and below another row. Drag a browser folder into the first, middle, and last pin gaps.
3. Drop a file onto a pinned folder body, then restart Strata to check the saved pin/places order.

Expected result: Compact rows, clear accent insertion lines at the intended position, persistent reordering, and unchanged file transfers onto folder bodies. Built-in places and pins cannot reorder into each other's section.

Related issue

Closes #1707

[l0gicgate]

Review

No merge blockers found. No commits pushed: the head (cf3a5ffe) already contains the latest main (4ec310dc).

What I checked

• SidebarDropFeedback ownership: a late pin validation, or a leave from a site the pointer already left, can't restore or clear the active indicator. Entering a new site resets the old one and cancels its spring-load. Sidebar rebuilds and the reorder drag_end both reset feedback.
• Gap layout: append_sidebar_row and insert_reorder_gap never stack two gaps. Each place row's before/after gap reorders with a fixed after. Pinned rows reuse the existing pin gaps.
• sidebar_reorder_matches keeps built-in places and pins in their own sections. Same-row drops onto a row's own gap are no-ops.
• Picker (local_only): FolderDragCache::new(true) rejects non-native pins before any probe, and remote pins stay hidden.
• Comments: the PR adds no code comments. The two E2E time.sleep comments explain a non-obvious wait, so they stay.
• Tests: the new unit and E2E tests cover distinct routes (row vs. gap, before vs. after, the section boundary, resize mid-drag, spring-load departure, persistence across restart). I found no tautological or layout assertions.

Scope note (not blocking): besides gap spacing, the picker sidebar now has the full place/pin behavior: reordering, drag-to-pin, and the Unpin/Customize…/Properties context menu. Unpinning Home or an XDG place there hides it everywhere, because it writes the shared preference. docs/portal-file-chooser.md documents this as intended.

Local verification (native, private headless display/bus)

• cargo fmt --all --check: pass
• cargo clippy --all-targets --all-features -- -D warnings: pass
• ./scripts/test-headless.py (full cargo test --all-targets --all-features): pass, lib 2805 passed / 0 failed / 9 ignored, all other targets green
• I didn't rerun E2E locally because I changed nothing. CI's E2E shards are green on this head. I didn't do a manual UI check; the PR's video covers the visuals.