fix(mobile): channel section folders bleed across linked workspaces

## Summary

On mobile, custom channel folders (sections) such as `production` / `estimates` created in one community remain visible after switching to a different linked workspace (e.g. Coker Construction → Lit Box), even when none of that community's channels exist there. Folders appear **empty** on the other workspace.

Desktop does **not** have this problem: #1477 scoped channel sections local storage to **pubkey + relay URL**. Mobile never received that port and still keys prefs account-globally.

## Environment

- Client: **Buzz mobile**
- Surfaces: channel list / sidebar sections
- Repro path: multiple communities linked on one account, switch between them

## Steps to reproduce

1. In community A (e.g. Coker Construction), create custom sections (e.g. `production`, `estimates`) and assign channels into them.
2. Link / switch to community B (e.g. Lit Box) on the same account.
3. Observe the channel list on B.

## Expected

Section folders are **per workspace/relay** (Desktop behavior after #1477). Community B only shows sections created for B.

## Actual

Section names from A appear on B, empty of channels. Layout is account-global on mobile.

## Root cause (code)

| Client | Local storage key |
|--------|-------------------|
| Desktop | `buzz-channel-sections.v1:$pubkey:$normalizedRelay` (#1477) |
| Mobile (before fix) | `buzz.channel-sections.v1:$pubkey` only |

Mobile also always renders every section even when zero channels resolve on the active relay.

Same intentional pattern already used on mobile for **channel-sort** (`channel_sort_storage.dart`).

## Fix

PR: https://github.com/block/buzz/pull/5761

- Scope mobile channel-sections SharedPreferences by pubkey + normalized relay URL
- One-time migration: first active relay after upgrade claims the legacy unscoped blob, then deletes the legacy key so later communities stay clean
- Provider remounts on active community change

## Related

- Desktop fix: #1477 `fix(sidebar): scope channel sections storage to relay URL`
- Adjacent (not this bug): #4883 section sync mobile ↔ desktop; #5469 shared team section layouts

## Release notes draft

> **Mobile:** Custom channel folders no longer leak empty across linked workspaces. Folders are now per community, matching Desktop.

## Verification

- [x] Unit/storage tests on PR #5761 (15/15 channel_sections suite)
- [ ] Manual: create folders on A, switch to B → A folders absent on B
- [ ] Manual: after upgrade, open original community first if you need legacy folders preserved there