refactor: remove UI initialisation callbacks

[matthewwalsh0]

Description

Replace legacy callback chain in UI initialisation with promises and async functions to improve readability and simplify maintenance.

Also removes some duplication in MV3 and legacy code.

Related issues

Manual testing steps

No functional changes.

Screenshots/Recordings

Before

After

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[matthewwalsh0]

My understanding is that we can now be certain the service worker will never restart while the browser is open, meaning this logic and the resetExtensionStreamAndListeners branch below is superfluous.

But for the sake of keeping this PR as a pure refactor with no functional changes, I've left it for now.

[metamaskbot]

Builds ready [2918681]

• builds: chrome, firefox
• builds (beta): chrome
• builds (flask): chrome, firefox
• builds (MMI): chrome, firefox
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Background Module Init Stats
• mv3: UI Init Stats
• mv3: Module Load Stats
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• code coverage: Report
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5
  • common: 0, 1, 10, 11, 2, 3, 4, 5, 6, 7, 8, 9
  • content-script: 0
  • offscreen: 0
  • ui: 0, 1, 10, 11, 2, 3, 4, 5, 6, 7, 8, 9

Page Load Metrics (1937 ± 246 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	277	3881	1796	713	342
		domContentLoaded	1489	3854	1905	504	242
		load	1497	3875	1937	512	246
		domInteractive	21	211	44	43	20

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: -200 Bytes (-0.00%)
• common: 0 Bytes (0.00%)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 94 lines in your changes missing coverage. Please review.

Project coverage is 70.19%. Comparing base (7ec7a45) to head (2918681).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
app/scripts/ui.js	0.00%	74 Missing ⚠️
ui/index.js	0.00%	20 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #26969      +/-   ##
===========================================
+ Coverage    70.17%   70.19%   +0.02%     
===========================================
  Files         1425     1425              
  Lines        49647    49639       -8     
  Branches     13889    13886       -3     
===========================================
+ Hits         34836    34842       +6     
+ Misses       14811    14797      -14     

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