chore: retain src input amount after switching tokens

[micaelae]

Description

Related issues

Fixes: https://consensyssoftware.atlassian.net/browse/MMS-1798

Manual testing steps

1. Load bridge page
2. Select amount, src token and dest token
3. Click token switch button to swap src and dest selections
4. Verify that src amount is preserved

Screenshots/Recordings

Before

Amount is cleared

After

Amount is preserved

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.

[metamaskbot]

Builds ready [1922b06]

• builds: chrome, firefox
• builds (flask): chrome, firefox
• builds (MMI): chrome
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

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

Page Load Metrics (1633 ± 45 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	349	1797	1565	294	141
		domContentLoaded	1415	1784	1609	90	43
		load	1419	1800	1633	94	45
		domInteractive	19	193	42	38	18
		backgroundConnect	5	62	23	19	9
		firstReactRender	16	88	45	26	13
		getState	4	49	13	14	7
		initialActions	0	1	0	0	0
		loadScripts	1054	1371	1211	75	36
		setupStore	6	52	9	10	5
		uiStartup	1602	2214	1908	150	72

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: -38 Bytes (-0.00%)
• common: 0 Bytes (0.00%)

[metamaskbot]

Builds ready [bd2ddf2]

• builds: chrome, firefox
• builds (flask): chrome, firefox
• builds (MMI): chrome
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

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

Page Load Metrics (1727 ± 164 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	1344	2916	1725	342	164
		domContentLoaded	1336	2875	1700	336	161
		load	1352	2913	1727	342	164
		domInteractive	23	212	54	42	20
		backgroundConnect	9	74	28	20	9
		firstReactRender	15	76	35	23	11
		getState	5	69	13	17	8
		initialActions	0	1	0	0	0
		loadScripts	937	2281	1253	290	139
		setupStore	6	67	21	21	10
		uiStartup	1512	3232	1962	375	180

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: -38 Bytes (-0.00%)
• common: 0 Bytes (0.00%)