chore: Update bottom border style for application header

[gambinish]

Description

Changes bottom shadow of app header to a border-bottom as issue specifies.

Related issues

Fixes: #27313

Manual testing steps

Screenshots/Recordings

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.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[metamaskbot]

Builds ready [9a6cbac]

• 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, 6
  • common: 0, 1, 10, 11, 12, 2, 3, 4, 5, 6, 7, 8, 9
  • content-script: 0
  • offscreen: 0
  • ui: 0, 1, 10, 11, 12, 13, 2, 3, 4, 5, 6, 7, 8, 9

Page Load Metrics (1777 ± 72 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	1494	2076	1768	159	76
		domContentLoaded	1483	2013	1745	149	71
		load	1525	2024	1777	149	72
		domInteractive	22	178	43	35	17
		backgroundConnect	10	78	36	26	12
		firstReactRender	16	91	42	27	13
		getState	5	22	9	4	2
		initialActions	0	1	0	0	0
		loadScripts	1004	1600	1286	156	75
		setupStore	6	68	22	21	10
		uiStartup	1768	2584	2070	215	103

Bundle size diffs

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

[gambinish]

This was complaining because the dot was obscuring the button

[FrederikBolding]

I'm really confused by this change, we use box-shadow for other headers too, e.g. the Snaps confirmation headers. If anything this will make that feel more disconnected. I thought we were trying to use the box-shadow look everywhere?

Perhaps @eriknson can chime in

[gambinish]

Here is the issue for reference, I'm open to adding this elsewhere if we need to: #27313