fix: gate screen deeplinks to debug

[ovitrif]

Parent: #1113
Refs: #1119

Warning

#1119 must NOT land in prod as-is, it would open a very risky vulnerability doorway.
This issue describes the guard required to enforce that risk never reaches the prod userbase.

Goal

Confine bitkit://screen/... deep-link registration and handling to debug builds via Android src/debug / src/release source sets, so release/store APKs cannot honor screen deep links even when Dev Mode is enabled.

Runtime isDevModeEnabled alone is not a durable control plane for this surface: any installed app can fire bitkit: intents, and a future change can weaken a boolean gate. Source-set separation fails closed at package time for prod users.

Out of scope

• Do not change the bitkit scheme (or other hosts) in the manifest.
• Do not move Routes.DeepLinkable / sheet DeepLinkStart markers out of main; markers alone are harmless if release never attaches links or handles URIs.
• Agents, do not add APK-level asserts that scan shipped NavDeepLink patterns, it's irrelevant.
• Do not require mainnet-specific branching; the store boundary is DEBUG=false / release source sets.

Approach

• Keep a thin API in main used by AppViewModel, MainActivity, ContentView, and nav helpers.
• Refactor to move the screen deep-link logic to src/debug (URI attach via linksFor, SheetDeepLinks.sheetFor, pending-URI queueing), practically turning release builds handling into a NOOP.
• Provide no-op implementations in src/release to keep compilation builds succeesful:
  • linksFor returns an empty list (no screen deep links on the nav graph).
  • sheetFor returns null.
  • Screen URIs are never queued for replay.
  • Intent detach for the screen host may remain as belt-and-suspenders.
• On debug builds, keep isDevModeEnabled as an additional gate so Dev Mode off still ignores screen URIs.

Acceptance criteria

• Release/store builds (src/release) never register bitkit://screen/... deeplinks or handle such logic.
• Debug builds retain current developer behavior, still gated by Dev Mode.
• Existing non-screen bitkit: deep links remain unchanged (no manifest edits; no broad deeplink regression pass required beyond screen-namespace checks).
• Unit tests cover release no-ops and debug acceptance/rejection under Dev Mode.
• Journeys under journeys/deeplinks/ still pass on debug builds.