style/standardize dapp casing

[Earnwithalee7890]

What changed? Why?
Standardized "dApp"/"dApps" to "dapp"/"dapps" (lowercase) across the documentation.
Modern blockchain documentation increasingly uses lowercase "dapp" as the term has become commonplace and no longer requires the capitalized "App" distinction. This aligns with Base's progressive, accessible documentation style.

Notes to reviewers

• Updated 7 files including provider documentation, gasless transactions guides, indexer tools, oracles, and wallet connection methods.
• Careful attention given to avoid changing instances within URLs or IDs (e.g., skipped Google Form ID containing "dApp" in base-services-hub.mdx line 35).
• Consistent lowercase usage improves readability and reduces visual clutter.

How has it been tested?

• grep search to identify all instances of "dApp" and "dApps" in documentation.
• Manual verification that URL paths, form IDs, and code identifiers were not modified.
• Confirmed changes render correctly in Mintlify preview.

[cb-heimdall]

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