feat: add configurable SSO login button text via ui.loginButtonText

[NotKiwy]

Closes #28248

Add ui.loginButtonText field to argocd-cm to allow overriding the SSO login button label. Falls back to the existing provider-derived label when not
set. Documentation updated in docs/operator-manual/custom-styles.md.

• Either (a) I've created an enhancement proposal and discussed it with the community, (b)
  this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Title of the PR
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on
  risk/complexity).

[bunnyshell]

✅ Preview Environment deployed on Bunnyshell

Component	Endpoints
argocd	https://argocd-twb3c7.bunnyenv.com/
argocd-ttyd	https://argocd-web-cli-twb3c7.bunnyenv.com/

See: Environment Details | Pipeline Logs

Available commands (reply to this comment):

• 🔴 /bns:stop to stop the environment
• 🚀 /bns:deploy to redeploy the environment
• ❌ /bns:delete to remove the environment

[codecov]

Bundle Report

Changes will increase total bundle size by 71 bytes (0.0%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
argo-cd-ui-array-push	10.67MB	71 bytes (0.0%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: argo-cd-ui-array-push

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
main.*.js	71 bytes	3.46MB	0.0%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@3336cf6). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #28254   +/-   ##
=========================================
  Coverage          ?   64.85%           
=========================================
  Files             ?      425           
  Lines             ?    59093           
  Branches          ?        0           
=========================================
  Hits              ?    38324           
  Misses            ?    17209           
  Partials          ?     3560           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.