[RFC] Consolidate two competing feature flag systems

## Problem

Two separate feature flag systems exist:

1. `src/composables/useFeatureFlags.ts` — fetches flags from the API based on platform distribution
2. `src/composables/useVueFeatureFlags.ts` — reads flags from the settings store

These have different sources of truth (API vs settings), different APIs, and different consumers. It is unclear when to use which.

## Proposed Fix

Options:
1. **Merge** into a single `useFeatureFlags` that combines both sources with clear precedence
2. **Differentiate clearly** — rename to `useRemoteFeatureFlags` and `useLocalFeatureFlags` with documentation on when to use each
3. **Unify** under a single feature flag service in `platform/` that abstracts the source

## Migration Plan

- Audit all consumers of both systems
- Choose approach based on usage patterns
- Single PR to consolidate or rename

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11095-RFC-Consolidate-two-competing-feature-flag-systems-33e6d73d3650815cbcd4dbea835e1a06) by [Unito](https://www.unito.io)